Optical amplifier evaluation method and optical amplifier evaluation device

Abstract

This invention is applied to an optical amplifier evaluation method of modulating by an optical modulator 23 light output from a light source 1 into a rectangular optical signal which is enabled/disabled in a predetermined period, then applying the optical signal to an optical fiber amplifier 2 to be measured, and obtaining the gain and noise figure of the optical fiber amplifier from the light intensities in the ON and OFF periods of an optical signal output from the optical fiber amplifier and the light intensity in the ON period of an optical signal input to the optical fiber amplifier. Output light from the optical fiber amplifier in a no-input state is passed through an optical path extending from the light source to the optical fiber amplifier and an optical path extending from the optical fiber amplifier to a light intensity measurement position, thereby obtaining optical losses on the respective optical paths. By the obtained optical losses, the light intensities are corrected. As a result, the gain G and noise figure NF of the optical fiber amplifier 2 can be attained with high precision.

What is claimed is:  
     
       1. An optical amplifier evaluation method comprising: 
       modulating light output from a light source into a rectangular optical signal having predetermined ON and OFF periods by a first optical modulator;  
       applying the optical signal modulated by said first optical modulator to an optical fiber amplifier to be evaluated;  
       passing the optical signal output from said optical fiber amplifier to be evaluated through a second optical modulator only during a given period in the OFF period of the optical signal modulated by said first optical modulator, thereby measuring a light intensity P ASE  of spontaneous emission in said optical fiber amplifier by a light intensity measurement device;  
       obtaining an optical loss on an optical path extending from said optical fiber amplifier to said light intensity measurement device using output light from said optical fiber amplifier in a no-input state, and correcting, using the obtained optical loss, the light intensity P ASE  of spontaneous emission in said optical fiber amplifier that is measured by said light intensity measurement device; and  
       obtaining a noise figure NF of an optical signal in said optical fiber amplifier using a corrected light intensity P ASE ′ of spontaneous emission in said optical fiber amplifier in accordance with the following equation:  
       
         
           NF=P ASE ′/(h·ν·G·Δν)  
         
       
       where 
       h: Planck's constant  
       ν: light frequency of input optical signal  
       G: gain  
       Δν: measurement frequency resolving power width (measurement frequency width) of said light intensity measurement device.  
     
     
       2. An optical amplifier evaluation method according to claim  1 , further comprising: 
       measuring a light intensity in the ON period of the optical signal input to said optical fiber amplifier by said light intensity measurement device;  
       measuring a light intensity in the ON period of an optical signal output from said optical fiber amplifier by said light intensity measurement device; and  
       obtaining the gain G of said optical fiber amplifier from the light intensities in the ON periods of the optical signals input to and output from said optical fiber amplifier that are measured by said light intensity measurement device.  
     
     
       3. An optical amplifier evaluation method comprising: 
       modulating light output from a light source into a rectangular optical signal having predetermined ON and OFF periods by a first optical modulator;  
       applying the optical signal modulated by said first optical modulator to an optical fiber amplifier to be evaluated;  
       passing the optical signal output from said optical fiber amplifier to be evaluated through a second optical modulator only during a given period in the OFF period of the optical signal modulated by said first optical modulator, thereby measuring a light intensity P ASE  of spontaneous emission in said optical fiber amplifier by a light intensity measurement device;  
       obtaining an optical loss on an optical path extending from said optical fiber amplifier to said light intensity measurement device using output light from said optical fiber amplifier in a no-input state, and correcting, using the obtained optical loss, the light intensity P ASE  of spontaneous emission in said optical fiber amplifier that is measured by said light intensity measurement device;  
       obtaining a noise figure NF of an optical signal in said optical fiber amplifier using a corrected light intensity P ASE ′ of spontaneous emission in said optical fiber amplifier in accordance with the following equation:  
       
         
           NF=P ASE ′/(h·ν·G·Δν)  
         
       
        where  
       h: Planck's constant  
       ν: light frequency of input optical signal  
       G: gain  
       Δν: measurement frequency resolving power width (measurement frequency width) of said light intensity measurement device; and  
       using an optical spectrum analyzer as said light intensity measurement device, and analyzing a spectrum of output light from said optical fiber amplifier in a no-input state by said optical spectrum analyzer, thereby obtaining a calibration value of a set frequency resolving power width used as the measurement frequency resolving power width (measurement frequency width) Δν of said light intensity measurement device from a ratio of level values of a spectrum for large and small set frequency resolving power widths.  
     
     
       4. An optical amplifier evaluation method comprising: 
       modulating light output from a light source into a rectangular optical signal having predetermined ON and OFF periods by a first optical modulator;  
       applying the optical signal modulated by said first optical modulator to an optical fiber amplifier to be evaluated;  
       passing the optical signal output from said optical fiber amplifier to be evaluated through a second optical modulator only during a given period in the OFF period of the optical signal modulated by said first optical modulator, thereby measuring a light intensity P ASE  of spontaneous emission in said optical fiber amplifier by a light intensity measurement device;  
       obtaining an optical loss on an optical path extending from said optical fiber amplifier to said light intensity measurement device using output light from said optical fiber amplifier in a no-input state, and correcting, using the obtained optical loss, the light intensity P ASE  of spontaneous emission in said optical fiber amplifier that is measured by said light intensity measurement device;  
       obtaining a noise figure NF of an optical signal in said optical fiber amplifier using a corrected light intensity P ASE ′ of spontaneous emission in said optical fiber amplifier in accordance with the following equation:  
       
         
           NF=P ASE ′/(h·ν·G·Δν)  
         
       
        where  
       h: Planck's constant  
       ν: light frequency of input optical signal  
       G: gain  
       Δν: measurement frequency resolving power width (measurement frequency width) of said light intensity measurement device;  
       measuring a light intensity in the ON period of the optical signal input to said optical fiber amplifier by said light intensity measurement device;  
       measuring a light intensity in the ON period of an optical signal output from said optical fiber amplifier by said light intensity measurement device;  
       obtaining the gain G of said optical fiber amplifier from the light intensities in the ON periods of the optical signals input to and output from said optical fiber amplifier that are measured by said light intensity measurement device;  
       obtaining an optical loss on an optical path extending from said light source to said optical fiber amplifier using output light from said optical fiber amplifier in a no-input state, and correcting, using the obtained optical loss, the light intensity in the ON period of the optical signal input to said optical fiber amplifier that is measured by said light intensity measurement device; and  
       obtaining an optical loss on an optical path extending from said optical fiber amplifier to the light intensity measurement position using output light from said optical fiber amplifier in a no-input state, and correcting, using the obtained optical loss, the light intensity in the ON period of the optical signal output from said optical fiber amplifier that is measured by said light intensity measurement device, and  
       wherein the gain G of said optical fiber amplifier is obtained from the corrected light intensities in the ON periods of the optical signals input to and output from said optical fiber amplifier.  
     
     
       5. An optical amplifier evaluation method comprising: 
       modulating light output from a light source into a rectangular optical signal having predetermined ON and OFF periods by a first optical modulator;  
       applying the optical signal modulated by said first optical modulator to an optical fiber amplifier to be evaluated;  
       passing the optical signal output from said optical fiber amplifier to be evaluated through a second optical modulator only during a given period in the OFF period of the optical signal modulated by said first optical modulator, thereby measuring a light intensity P ASEM  of spontaneous emission in said optical fiber amplifier by a light intensity measurement device;  
       obtaining an optical loss on an optical path extending from said optical fiber amplifier to said light intensity measurement device using output light from said optical fiber amplifier in a no-input state, and correcting, using the obtained optical loss, the light intensity P ASE  of spontaneous emission in said optical fiber amplifier that is measured by said light intensity measurement device;  
       obtaining a noise figure NF of an optical signal in said optical fiber amplifier using a corrected light intensity P ASE ′ of spontaneous emission in said optical fiber amplifier in accordance with the following equation:  
       
         
           NF=P ASE ′/(h·ν·G·Δν)  
         
       
        where  
       h: Planck's constant  
       ν: light frequency of input optical signal  
       G: gain  
       Δν: measurement frequency resolving power width (measurement frequency width) of said light intensity measurement device; and  
       measuring a light intensity in the ON period of the optical signal input to said optical fiber amplifier by said light intensity measurement device;  
       measuring a light intensity in the ON period of an optical signal output from said optical fiber amplifier by said light intensity measurement device;  
       obtaining the gain G of said optical fiber amplifier from the light intensities in the ON periods of the optical signals input to and output from said optical fiber amplifier that are measured by said light intensity measurement device; and  
       using an optical spectrum analyzer as said light intensity measurement device, and analyzing a spectrum of output light from said optical fiber amplifier in a no-input state by said optical spectrum analyzer, thereby obtaining a calibration value of a set frequency resolving power width used as the measurement frequency resolving power width (measurement frequency width) Δν of said light intensity measurement device from a ratio of level values of a spectrum for large and small set frequency resolving power widths.  
     
     
       6. An optical amplifier evaluation method according to claim  5 , wherein the step of obtaining the gain G of said optical fiber amplifier comprises: 
       obtaining an optical loss on an optical path extending from said light source to said optical fiber amplifier using output light from said optical fiber amplifier in a no-input state, and correcting, using the obtained optical loss, the light intensity in the ON period of the optical signal input to said optical fiber amplifier that is measured by said light intensity measurement device; and  
       obtaining an optical loss on an optical path extending from said optical fiber amplifier to the light intensity measurement position using output light from said optical fiber amplifier in a no-input state, and correcting, using the obtained optical loss, the light intensity in the ON period of the optical signal output from said optical fiber amplifier that is measured by said light intensity measurement device, and  
       wherein the gain G of said optical fiber amplifier is obtained from the corrected light intensities in the ON periods of the optical signals input to and output from said optical fiber amplifier.  
     
     
       7. An optical amplifier evaluation apparatus comprising: 
       a first optical modulator for modulating light output from a light source into a rectangular optical signal having predetermined ON and OFF periods;  
       the optical signal modulated by said first optical modulator being applied to an optical fiber amplifier to be evaluated;  
       a light intensity measurement device for passing the optical signal output from said optical fiber amplifier through a second optical modulator only during a given period in the OFF period of the optical signal modulated by said first optical modulator, thereby measuring a light intensity P ASE  of spontaneous emission in said optical fiber amplifier;  
       means for obtaining an optical loss on an optical path extending from said optical fiber amplifier to said light intensity measurement device using output light from said optical fiber amplifier in a no-input state, and correcting, using the obtained optical loss, the light intensity P ASE  of spontaneous emission in said optical fiber amplifier that is measured by said light intensity measurement device;  
       means for obtaining a noise figure NF of an optical signal in said optical fiber amplifier using a corrected light intensity P ASE ′ of spontaneous emission in said optical fiber amplifier in accordance with the following equation:  
       
         
           NF=P ASE ′/(h·ν·G·Δν)  
         
       
        where  
       h: Planck's constant  
       ν: light frequency of input optical signal  
       G: gain  
       Δν: measurement frequency resolving power width (measurement frequency width) of said light intensity measurement device;  
       said light intensity device measuring a light intensity in the ON period of the optical signal input to said optical fiber amplifier;  
       said light intensity device measuring a light intensity in the ON period of an optical signal output from said optical fiber amplifier;  
       means for obtaining the gain G of said optical fiber amplifier from the light intensities in the ON periods of the optical signals input to and output from said optical fiber amplifier that are measured by said light intensity measurement device;  
       wherein said light intensity measurement device comprises an optical spectrum analyzer which analyzes a spectrum of output light from said optical fiber amplifier in a no-input state by said optical spectrum analyzer, thereby obtaining a calibration value of a set frequency resolving power width used as the measurement frequency resolving power width (measurement frequency width) Δν of said light intensity measurement device from a ratio of level values of a spectrum for large and small set frequency resolving power widths;  
       means for obtaining an optical loss on an optical path extending from said light source to said optical fiber amplifier using output light from said optical fiber amplifier in a no-input state, and correcting, using the obtained optical loss, the light intensity in the ON period of the optical signal input to said optical fiber amplifier that is measured by said light intensity measurement device;  
       means for obtaining an optical loss on an optical path extending from said optical fiber amplifier to the light intensity measurement position using output light from said optical fiber amplifier in a no-input state, and correcting, using the obtained optical loss, the light intensity in the ON period of the optical signal output from said optical fiber amplifier that is measured by said light intensity measurement device;  
       wherein the gain G of said optical fiber amplifier is obtained from the corrected light intensities in the ON periods of the optical signals input to and output from said optical fiber amplifier;  
       switching means arranged between a first terminal for receiving an optical output from said light source, an input terminal of said first optical modulator, an output terminal of said first optical modulator, an input terminal of said second optical modulator, an output terminal of said second optical modulator, an output terminal to said optical fiber amplifier, an input terminal from said optical fiber amplifier, and an output terminal to said light intensity measurement device; and  
       control means for measuring the light intensity P ASE  of spontaneous emission in said optical fiber amplifier by a first switching operation of said switching means, measuring the gain G of said optical fiber amplifier by a second switching operation, and measuring the measurement frequency resolving power width (measurement frequency width) Δν of said light intensity measurement device by a third switching operation.  
     
     
       8. An optical amplifier evaluation apparatus according to claim  7 , wherein said switching means comprises: 
       first and second optical switches, and wherein  
       each of said first and second optical switches has a total of four, first to fourth terminals, the first and second terminals and the third and fourth terminals of each switch being connected in a normal state, and wherein said first and second optical switches switch between “steady state” and “switching state” in accordance with an instruction from said control means.  
     
     
       9. An optical amplifier evaluation apparatus according to claim  8 , wherein said control means stores in advance a measurement value of a reference light intensity P ref  at each wavelength λ obtained by directly connecting said optical spectrum analyzer to an output terminal of said optical fiber amplifier in a no-input state and analyzing a spectrum of output light serving as reference light output from said optical fiber amplifier in a no-input state. 
     
     
       10. An optical amplifier evaluation apparatus according to claim  9 , wherein said control means applies output light from said optical fiber amplifier in a no-input state to the input terminal to said first optical modulator, sets said first optical switch to the “steady state”, and connects said optical spectrum analyzer to the output terminal of said optical spectrum analyzer in the “steady state”, thereby measuring a light intensity Pa(λ) at each wavelength λ of the optical signal having passed from said optical fiber amplifier through an optical path including said first optical modulator and said first optical switch, 
       an optical loss La(λ) on the optical path is obtained and stored in said control means using the light intensity P ref (λ) of reference light stored in said control means in accordance with the following equation:  
       
         
           La(λ)=Pa(λ)/P ref (λ)  
         
       
       said control means applies output light from said optical fiber amplifier in a no-input state to the input terminal to said first optical modulator, sets said first and second optical switches to the “switching state”, and connects said optical spectrum analyzer to the output terminal to said optical spectrum analyzer in the “switching state”, thereby measuring a light intensity Pd(λ) at each wavelength λ of the optical signal having passed from said optical fiber amplifier through an optical path including said first optical modulator and said first and second optical switches, and  
       an optical loss Ld(λ) on the optical path is obtained and stored in said control means using the light intensity P ref (λ) of reference light stored in said control means in accordance with the following equation:  
       
         
           Ld(λ)=Pd(λ)/P ref (λ).  
         
       
     
     
       11. An optical amplifier evaluation apparatus according to claim  9 , wherein said control means applies output light from said optical fiber amplifier in a no-input state to the input terminal to said optical fiber amplifier, sets said first optical switch to the “steady state”, sets said second optical switch to the “switching state”, and connects said optical spectrum analyzer to the output terminal to said optical spectrum analyzer in this state, thereby measuring a light intensity Pb(λ) at each wavelength λ of the optical signal having passed from said optical fiber amplifier through an optical path including only said first and second optical switches, and 
       an optical loss Lb(λ) on the optical path not including said second optical modulator is obtained and stored in said control means using the light intensity P ref (λ) of reference light stored in said control means in accordance with the following equation:  
       
         
           Lb(λ)=Pb(λ)/P ref (λ).  
         
       
     
     
       12. An optical amplifier evaluation apparatus according to claim  9 , wherein said control means applies output light from said optical fiber amplifier in a no-input state to the input terminal to said optical fiber amplifier, sets said first and second optical switches to the “steady state”, and connects said optical spectrum analyzer to the output terminal to said optical spectrum analyzer in the “steady state”, thereby measuring a light intensity Pc(λ) at each wavelength λ of the optical signal having passed from said optical fiber amplifier through an optical path including said first and second optical switches and said second optical modulator, and 
       an optical loss Lc(λ) on the optical path including said second optical modulator is obtained and stored in said control means using the light intensity P ref (λ) of reference light stored in said control means in accordance with the following equation:  
       
         
           Lc(λ)=Pc(λ)/P ref (λ).  
         
       
     
     
       13. An optical amplifier evaluation apparatus according to claim  10 , wherein said control means sets said first and second optical switches to the “switching state”, and sends a light intensity measurement command to said optical spectrum analyzer in the “switching state” to analyze a spectrum of incident light, thereby obtaining a light intensity P INM  at each wavelength λ, and 
       the light intensity P INM  is corrected using the optical losses Ld(λ) and La(λ) stored in said control means in accordance with the following equation:  
       
         
           P IN (λ)=P INM (λ)·La(λ)/Ld(λ)  
         
       
       thereby obtaining a correct input light intensity P IN (λ) to said optical fiber amplifier. 
     
     
       14. An optical amplifier evaluation apparatus according to claim  11 , wherein said control means sets said first optical switch to the “steady state”, sets said second optical switch to the “switching state”, and sends a light intensity measurement instruction to said optical spectrum analyzer in this state to spectrum-analyze incident light, thereby obtaining a light intensity P OUTM  at each wavelength λ, and 
       the light intensity P OUTM  is corrected using the optical loss Lb(λ) stored in said control means in accordance with the following equation:  
       
         
           P OUT (λ)=P OUTM (λ)/Lb(λ)  
         
       
       thereby obtaining a correct output light intensity P OUT (λ) from said optical fiber amplifier. 
     
     
       15. An optical amplifier evaluation apparatus according to claim  12 , wherein said control means sets said first and second optical switches to the “steady state”, and sends a light intensity measurement instruction to said optical spectrum analyzer in the “steady state”, thereby obtaining a light intensity P ASEM  at each wavelength λ of an optical signal as spontaneous emission (ASE) in a partial period T A  of the OFF period of the input, amplified optical signal, and 
       the light intensity P ASEM  is corrected using the optical loss Lc(λ) stored in said control means in accordance with the following equation:  
        P ASE (λ)=P ASEM (λ)/Lc(λ) 
       thereby obtaining a correct input light intensity P ASE (λ) of spontaneous emission (ASE) in said optical fiber amplifier. 
     
     
       16. An optical amplifier evaluation apparatus according to claim  9 , wherein said control means applies output light from said optical fiber amplifier in a no-input state to the input terminal to said first optical modulator, sets said first optical switch to the “steady state”, and connects said optical spectrum analyzer to the output terminal to said optical spectrum analyzer in the “steady state”, thereby measuring a light intensity Pa(λ) at each wavelength λ of the optical signal having passed from said optical fiber amplifier through an optical path including said first optical modulator and said first optical switch, 
       an optical loss La(λ) on the optical path is obtained and stored in said control means using the light intensity P ref (λ) of reference light stored in said control means in accordance with the following equation:  
       
         
           La(λ)=Pa(λ)/P ref (λ)  
         
       
       said control means applies output light from said optical fiber amplifier in a no-input state to the input terminal to said first optical modulator, sets said first and second optical switches to the “switching state”, and connects said optical spectrum analyzer to the output terminal to said optical spectrum analyzer in the “switching state”, thereby measuring a light intensity Pd(λ) at each wavelength λ of the optical signal having passed from said optical fiber amplifier through an optical path including said first optical modulator and said first and second optical switches,  
       an optical loss Ld(λ) on the optical path is obtained and stored in said control means using the light intensity P ref (λ) of reference light stored in said control means in accordance with the following equation:  
       
         
           Ld(λ)=Pd(λ)/P ref (λ)  
         
       
       said control means applies output light from said optical fiber amplifier in a no-input state to the input terminal to said optical fiber amplifier, sets said first optical switch to the “steady state”, sets said second optical switch to the “switching state”, and connects said optical spectrum analyzer to the output terminal to said optical spectrum analyzer in this state, thereby measuring a light intensity Pb(λ) at each wavelength λ of the optical signal having passed from said optical fiber amplifier through an optical path including only said first and second optical switches,  
       an optical loss Lb(λ) on the optical path not including said second optical modulator is obtained and stored in said control means using the light intensity P ref (λ) of reference light stored in said control means in accordance with the following equation:  
       
         
           Lb(λ)=Pb(λ)/P ref (λ)  
         
       
       said control means applies output light from said optical fiber amplifier in a no-input state to the input terminal to said optical fiber amplifier, sets said first and second optical switches to the “steady state”, and connects said optical spectrum analyzer to the output terminal to said optical spectrum analyzer in the “steady state”, thereby measuring a light intensity Pc(λ) at each wavelength λ of the optical signal having passed from said optical fiber amplifier through an optical path including said first and second optical switches and said second optical modulator, and  
       an optical loss Lc(λ) on the optical path including said second optical modulator is obtained and stored in said control means using the light intensity P ref (λ) of reference light stored in said control means in accordance with the following equation:  
       
         
           Lc(λ)=PC(λ)/P ref (λ).  
         
       
     
     
       17. An optical amplifier evaluation apparatus according to claim  16 , wherein said control means sets said first and second optical switches to the “switching state”, and sends a light intensity measurement command to said optical spectrum analyzer in the “switching state” to analyze a spectrum of incident light, thereby obtaining a light intensity P INM  at each wavelength λ, 
       the light intensity P INM  is corrected using the optical losses Ld(λ) and La(λ) stored in said control means in accordance with the following equation:  
       
         
           P IN (λ)=P INM (λ)·La(λ)/Ld(λ)  
         
       
       thereby obtaining a correct input light intensity P IN (λ) to said optical fiber amplifier, 
       said control means sets said first optical switch to the “steady state”, sets said second optical switch to the “switching state”, and sends a light intensity measurement instruction to said optical spectrum analyzer in this state to spectrum-analyze incident light, thereby obtaining a light intensity P OUTM  at each wavelength λ,  
       the light intensity P OUTM  is corrected using the optical loss Lb(λ) stored in said control means in accordance with the following equation:  
       
         
           P OUT (λ)=P OUTM (λ)/Lb(λ)  
         
       
       thereby obtaining a correct output light intensity P OUT (λ) from said optical fiber amplifier, 
       said control means sets said first and second optical switches to the “steady state”, and sends a light intensity measurement instruction to said optical spectrum analyzer in the “steady state”, thereby obtaining a light intensity P ASEM  at each wavelength λ of an optical signal as spontaneous emission (ASE) in a partial period T A  of the OFF period of the input, amplified optical signal, and  
       the light intensity P ASEM  is corrected using the optical loss Lc(λ) stored in said control means in accordance with the following equation:  
       
         
           P ASE (λ)=P ASEM (λ)/Lc(λ)  
         
       
       thereby obtaining a correct input light intensity P ASE (λ) of spontaneous emission (ASE) in said optical fiber amplifier. 
     
     
       18. An optical amplifier evaluation apparatus according to claim  17 , wherein a gain G(λ) and noise figure NF(λ) of said optical fiber amplifier are calculated using the corrected input light intensity P IN (λ), output light intensity P OUT (λ), and light intensity P ASE (λ) of spontaneous emission (ASE) in accordance with following equations: 
       
         
           G(λ)=P OUT (λ)/P IN (λ),  
         
       
       
         
           NF(λ)=P ASE (λ)/[h·ν·G·Δν] 
         
       
       where 
       h: Planck's constant  
       ν: light frequency of input optical signal  
       G: gain  
       Δν: measurement frequency resolving power width (measurement frequency width) of light intensity measurement device.  
     
     
       19. An optical amplifier evaluation apparatus according to claim  18 , characterized in that, in the “steady state” by a command from said control means, said first optical switch inputs an optical signal having passed through the first and second terminals to an input terminal of said optical fiber amplifier via the output terminal to said optical fiber amplifier, and inputs the amplified optical signal output from an output terminal of said optical fiber amplifier to the third terminal via the input terminal from said optical fiber amplifier and then to the first terminal of said second optical switch via the fourth terminal, and 
       in the “steady state” by a command from said control means, said second optical switch inputs the optical signal input to the first terminal, to said second optical modulator via the second terminal and the input terminal to said second optical modulator, inputs the modulated optical signal output from said second optical modulator to the third terminal via the output terminal from said second optical modulator, and inputs the optical signal input to the third terminal, to said external optical spectrum analyzer serving as said light intensity measurement via the fourth terminal and the output terminal to said light intensity measurement device, thereby allowing to measure the light intensity P ASE  of spontaneous emission in said optical fiber amplifier by the first switching operation.  
     
     
       20. An optical amplifier evaluation apparatus according to claim  7 , wherein said switching means comprises a total of eight terminals, i.e., an input terminal from said light source, an output terminal to said optical fiber amplifier, an input terminal from said optical fiber amplifier, an output terminal to said optical spectrum analyzer, an input terminal to said first optical modulator, an output terminal from said first optical modulator, an input terminal to said second optical modulator, and an output terminal from said second optical modulator, and 
       connection between the terminals assembled in said switching means is arbitrarily switched by said control means.  
     
     
       21. An optical amplifier evaluation apparatus according to claim  7 , wherein said switching means comprises a first optical switch arranged between an input terminal from said light source and an input terminal to said first optical modulator, a second optical switch arranged between an output terminal from said first optical modulator and an output terminal to said optical fiber amplifier, a third optical switch arranged between an input terminal from said optical fiber amplifier and an input terminal to said second optical modulator, and a fourth optical switch arranged between an output terminal from said second optical modulator and an output terminal to said optical spectrum analyzer, 
       each of said first and second optical switches has a total of three, ports  0  to  2  switched by said control means, and  
       each of said third and fourth optical switches has a total of four, ports  0  to  3  switched by said control means.  
     
     
       22. An optical amplifier evaluation method comprising: 
       modulating light output from a light source into a rectangular optical signal having predetermined ON and OFF periods by a first optical modulator;  
       applying the optical signal modulated by said first optical modulator to an optical fiber amplifier to be evaluated;  
       passing the optical signal output from said optical fiber amplifier to be evaluated through a second optical modulator only during a given period in the OFF period of the optical signal modulated by said first optical modulator, thereby measuring a light intensity P ASE  of spontaneous emission in said optical fiber amplifier by a light intensity measurement device;  
       obtaining an optical loss on an optical path extending from said optical fiber amplifier to said light intensity measurement device using output light from an unpolarized light generator, in a no-input state, and correcting, using the obtained optical loss, the light intensity P ASE  of spontaneous emission in said optical fiber amplifier that is measured by said light intensity measurement device;  
       obtaining a noise figure NF of an optical signal in said optical fiber amplifier using a corrected light intensity P ASE ′ of spontaneous emission in said optical fiber amplifier in accordance with the following equation:  
       
         
           NF=P ASE ′/(h·ν·G·Δν)  
         
       
        where  
       h: Planck's constant  
       ν: light frequency of input optical signal  
       G: gain  
       Δν: measurement frequency resolving power width (measurement frequency width) of said light intensity measurement device; and  
       using an optical spectrum analyzer as said light intensity measurement device, and analyzing a spectrum of output light from said unpolarized light generator by said optical spectrum analyzer, thereby obtaining a calibration value of a set frequency resolving power width used as the measurement frequency resolving power width (measurement frequency width) Δν of said light intensity measurement device from a ratio of level values of a spectrum for large and small set frequency resolving power widths.  
     
     
       23. An optical amplifier evaluation method comprising: 
       modulating light output from a light source into a rectangular optical signal having predetermined ON and OFF periods by a first optical modulator;  
       applying the optical signal modulated by said first optical modulator to an optical fiber amplifier to be evaluated;  
       passing the optical signal output from said optical fiber amplifier to be evaluated through a second optical modulator only during a given period in the OFF period of the optical signal modulated by said first optical modulator, thereby measuring a light intensity P ASE  of spontaneous emission in said optical fiber amplifier by a light intensity measurement device;  
       obtaining an optical loss on an optical path extending from said optical fiber amplifier to said light intensity measurement device using output light from an unpolarized light generator, in a no-input state, and correcting, using the obtained optical loss, the light intensity P ASE  of spontaneous emission in said optical fiber amplifier that is measured by said light intensity measurement device;  
       obtaining a noise figure NF of an optical signal in said optical fiber amplifier using a corrected light intensity P ASE ′ of spontaneous emission in said optical fiber amplifier in accordance with the following equation:  
       
         
           NF=P ASE ′/(h·ν·G·Δν)  
         
       
        where  
       h: Planck's constant  
       ν: light frequency of input optical signal  
       G: gain  
       Δν: measurement frequency resolving power width (measurement frequency width) of said light intensity measurement device;  
       measuring a light intensity in the ON period of the optical signal input to said optical fiber amplifier by said light intensity measurement device;  
       measuring a light intensity in the ON period of an optical signal output from said optical fiber amplifier by said light intensity measurement device;  
       obtaining the gain G of said optical fiber amplifier from the light intensities in the ON periods of the optical signals input to and output from said optical fiber amplifier that are measured by said light intensity measurement device;  
       obtaining an optical loss on an optical path extending from said light source to said optical fiber amplifier using output light from said unpolarized light generator, and correcting, using the obtained optical loss, the light intensity in the ON period of the optical signal input to said optical fiber amplifier that is measured by said light intensity measurement device; and  
       obtaining an optical loss on an optical path extending from said optical fiber amplifier to the light intensity measurement position using output light from said unpolarized light generator, and correcting, using the obtained optical loss, the light intensity in the ON period of the optical signal output from said optical fiber amplifier that is measured by said light intensity measurement device, and  
       wherein the gain G of said optical fiber amplifier is obtained from the corrected light intensities in the ON periods of the optical signals input to and output from said optical fiber amplifier.  
     
     
       24. An optical amplifier evaluation method comprising: 
       modulating light output from a light source into a rectangular optical signal having predetermined ON and OFF periods by a first optical modulator;  
       applying the optical signal modulated by said first optical modulator to an optical fiber amplifier to be evaluated;  
       passing the optical signal output from said optical fiber amplifier to be evaluated through a second optical modulator only during a given period in the OFF period of the optical signal modulated by said first optical modulator, thereby measuring a light intensity P ASE  of spontaneous emission in said optical fiber amplifier by a light intensity measurement device;  
       obtaining an optical loss on an optical path extending from said optical fiber amplifier to said light intensity measurement device using output light from an unpolarized light generator, in a no-input state, and correcting, using the obtained optical loss, the light intensity P ASEM  of spontaneous emission in said optical fiber amplifier that is measured by said light intensity measurement device;  
       obtaining a noise figure NF of an optical signal in said optical fiber amplifier using a corrected light intensity P ASE  of spontaneous emission in said optical fiber amplifier in accordance with the following equation:  
       
         
           NF=P ASE ′/(h·ν·G·Δν)  
         
       
        where  
       h: Planck's constant  
       ν: light frequency of input optical signal  
       G: gain  
       Δν: measurement frequency resolving power width (measurement frequency width) of said light intensity measurement device;  
       measuring a light intensity in the ON period of the optical signal input to said optical fiber amplifier by said light intensity measurement device;  
       measuring a light intensity in the ON period of an optical signal output from said optical fiber amplifier by said light intensity measurement device;  
       obtaining the gain G of said optical fiber amplifier from the light intensities in the ON periods of the optical signals input to and output from said optical fiber amplifier that are measured by said light intensity measurement device;  
       using an optical spectrum analyzer as said light intensity measurement device, and analyzing a spectrum of output light from said unpolarized light generator by said optical spectrum analyzer, thereby obtaining a calibration value of a set frequency resolving power width used as the measurement frequency resolving power width (measurement frequency width) Δν of said light intensity measurement device from a ratio of level values of a spectrum for large and small set frequency resolving power widths.  
     
     
       25. An optical amplifier evaluation method according to claim  24 , wherein the step of obtaining the gain G of said optical fiber amplifier comprises: 
       obtaining an optical loss on an optical path extending from said light source to said optical fiber amplifier using output light from said unpolarized light generator, and correcting, using the obtained optical loss, the light intensity in the ON period of the optical signal input to said optical fiber amplifier that is measured by said light intensity measurement device; and  
       obtaining an optical loss on an optical path extending from said optical fiber amplifier to the light intensity measurement position using output light from said unpolarized light generator, and correcting, using the obtained optical loss, the light intensity in the ON period of the optical signal output from said optical fiber amplifier that is measured by said light intensity measurement device, and  
       the wherein gain G of said optical fiber amplifier is obtained from the corrected light intensities in the ON periods of the optical signals input to and output from said optical fiber amplifier.  
     
     
       26. An optical amplifier evaluation apparatus comprising: 
       a first optical modulator for modulating light output from a light source into a rectangular optical signal having predetermined ON and OFF periods by a first optical modulator;  
       the optical signal modulated by said first optical modulator being applied to an optical fiber amplifier to be evaluated;  
       a light intensity measuring device for passing the optical signal output from said optical fiber amplifier to be evaluated through a second optical modulator only during a given period in the OFF period of the optical signal modulated by said first optical modulator, thereby measuring a light intensity P ASE  of spontaneous emission in said optical fiber amplifier by a light intensity measurement device;  
       means for obtaining an optical loss on an optical path extending from said optical fiber amplifier to said light intensity measurement device using output light from an unpolarized light generator, in a no-input state, and correcting, using the obtained optical loss, the light intensity P ASE  of spontaneous emission in said optical fiber amplifier that is measured by said light intensity measurement device;  
       means for obtaining a noise figure NF of an optical signal in said optical fiber amplifier using a corrected light intensity of spontaneous emission in said optical fiber amplifier in accordance with the following equation:  
        NF=P ASE ′/(h·ν·G·Δν) 
        where  
       h: Planck's constant  
       ν: light frequency of input optical signal  
       G: gain  
       Δν: measurement frequency resolving power width (measurement frequency width) of said light intensity measurement device;  
       said light intensity measuring device measuring a light intensity in the ON period of the optical signal input to said optical fiber amplifier by said light intensity measurement device;  
       said light intensity measuring device measuring a light intensity in the ON period of an optical signal output from said optical fiber amplifier by said light intensity measurement device;  
       means for obtaining the gain G of said optical fiber amplifier from the light intensities in the ON periods of the optical signals input to and output from said optical fiber amplifier that are measured by said light intensity measurement device;  
       wherein said light intensity measuring device comprises an optical spectrum analyzer which analyzes a spectrum of output light from said unpolarized light generator by said optical spectrum analyzer, thereby obtaining a calibration value of a set frequency resolving sower width used as the measurement frequency resolving power width (measurement frequency width) Δν of said light intensity measurement device from a ratio of level values of a spectrum for large and small set frequency resolving power widths;  
       means for obtaining an optical loss on an optical path extending from said light source to said optical fiber amplifier using output light from said unpolarized light generator, and correcting using the obtained optical loss, the light intensity in the ON period of the optical signal input to said optical fiber amplifier that is measured by said light intensity measurement device;  
       means for obtaining an optical loss on an optical path extending from said optical fiber amplifier to the light intensity measurement position using output light from said unpolarized light generator, and correcting, using the obtained optical loss, the light intensity in the ON period of the optical signal output from said optical fiber amplifier that is measured by said light intensity measurement device,  
       wherein the gain G of said optical fiber amplifier is obtained from the corrected light intensities in the ON periods of the optical signals input to and output from said optical fiber amplifier;  
       switching means arranged between a first terminal for receiving an optical output from said light source, an input terminal of said first optical modulator, an output terminal of said first optical modulator, an input terminal of said second optical modulator, an output terminal of said second optical modulator, an output terminal to said optical fiber amplifier, an input terminal from said optical fiber amplifier, and an output terminal to said light intensity measurement device; and  
       control means for measuring the light intensity P ASE  of spontaneous emission in said optical fiber amplifier by a first switching operation of said switching means, measuring the gain G of said optical fiber amplifier by a second switching operation, and measuring the measurement frequency resolving power width (measurement frequency width) Δν of said light intensity measurement device by a third switching operation.  
     
     
       27. An optical amplifier evaluation apparatus according to claim  26 , wherein said switching means comprises: 
       first and second optical switches, and  
       each of said first and second optical switches has a total of four, first to fourth terminals, the first and second terminals and the third and fourth terminals of each switch being connected in a normal state, and said first and second optical switches switch between “steady state” and “switching state” in accordance with an instruction from said control means.  
     
     
       28. An optical amplifier evaluation apparatus according to claim  27 , wherein said control means stores in advance a measurement value of a reference light intensity P ref  at each wavelength λ obtained by directly connecting said optical spectrum analyzer to an output terminal of an unpolarized light generator and analyzing a spectrum of output light serving as reference light output from said unpolarized light generator. 
     
     
       29. An optical amplifier evaluation apparatus according to claim  28 , wherein said control means applies output light from said unpolarized light generator to the input terminal to said first optical modulator, sets said first optical switch to the “steady state”, and connects said optical spectrum analyzer to the output terminal to said optical spectrum analyzer in the “steady state”, thereby measuring a light intensity Pa(λ) at each wavelength λ of the optical signal having passed from said optical fiber amplifier through an optical path including said first optical modulator and said first optical switch, 
       an optical loss La(λ) on the optical path is obtained and stored in said control means using the light intensity P ref (λ) of reference light stored in said control means in accordance with the following equation:  
       
         
           La(λ)=Pa(λ)/P ref (λ).  
         
       
       said control means applies output light from said unpolarized light generator to the input terminal to said first optical modulator, sets said first and second optical switches to the “switching state”, and connects said optical spectrum analyzer to the output terminal to said optical spectrum analyzer in the “switching state”, thereby measuring a light intensity Pd(λ) at each wavelength λ of the optical signal having passed from said optical fiber amplifier through an optical path including said first optical modulator and said first and second optical switches, and  
       an optical loss Ld(λ) on the optical path is obtained and stored in said control means using the light intensity P ref (λ) of reference light stored in said control means in accordance with the following equation:  
       
         
           Ld(λ)=Pd(λ)/P ref (λ).  
         
       
     
     
       30. An optical amplifier evaluation apparatus according to claim  28 , wherein said control means applies output light from said unpolarized light generator to the input terminal to said optical fiber amplifier, sets said first optical switch to the “steady state”, sets said second optical switch to the “switching state”, and connects said optical spectrum analyzer to the output terminal to said optical spectrum analyzer in this state, thereby measuring a light intensity Pb(λ) at each wavelength λ of the optical signal having passed from said optical fiber amplifier through an optical path including only said first and second optical switches, and 
       an optical loss Lb(λ) on the optical path not including said second optical modulator is obtained and stored in said control means using the light intensity P ref (λ) of reference light stored in said control means in accordance with the following equation:  
       
         
           Lb(λ)=Pb(λ)/P ref (λ).  
         
       
     
     
       31. An optical amplifier evaluation apparatus according to claim  28 , wherein said control means applies output light from said unpolarized light generator to the input terminal to said optical fiber amplifier, sets said first and second optical switches to the “steady state”, and connects said optical spectrum analyzer to the output terminal to said optical spectrum analyzer in the “steady state”, thereby measuring a light intensity Pc(λ) at each wavelength λ of the optical signal having passed from said optical fiber amplifier through an optical path including said first and second optical switches and said second optical modulator, and 
       an optical loss Lc(λ) on the optical path including said second optical modulator is obtained and stored in said control means using the light intensity P ref (λ) of reference light stored in said control means in accordance with the following equation:  
       
         
           Lc(λ)=Pc(λ)/P ref (λ).  
         
       
     
     
       32. An optical amplifier evaluation apparatus according to claim  29 , wherein said control means sets said first and second optical switches to the “switching state”, and sends a light intensity measurement command to said optical spectrum analyzer in the “switching state” to analyze a spectrum of incident light, thereby obtaining a light intensity P INM  at each wavelength λ, and 
       the light intensity P INM  is corrected using the optical losses Ld(λ) and La(λ) stored in said control means in accordance with the following equation:  
       
         
           P IN (λ)=P INM (λ)·La(λ)/Ld(λ)  
         
       
       thereby obtaining a correct input light intensity P IN (λ) to said optical fiber amplifier. 
     
     
       33. An optical amplifier evaluation apparatus according to claim  30 , wherein said control means sets said first optical switch to the “steady state”, sets said second optical switch to the “switching state”, and sends a light intensity measurement instruction to said optical spectrum analyzer in this state to spectrum-analyze incident light, thereby obtaining a light intensity P OUTM  at each wavelength λ, and 
       the light intensity P OUTM  is corrected using the optical loss Lb(λ) stored in said control means in accordance with the following equation:  
       
         
           P OUT (λ)=P OUTM (λ)/Lb(λ)  
         
       
       thereby obtaining a correct input light intensity P OUT (λ) to said optical fiber amplifier. 
     
     
       34. An optical amplifier evaluation apparatus according to claim  31 , wherein said control means sets said first optical switch to the “steady state”, sets said second optical switch to the “switching state”, and sends a light intensity measurement instruction to said optical spectrum analyzer in this state to spectrum-analyze incident light, thereby obtaining a light intensity P OUTM  at each wavelength λ, and 
       the light intensity P OUTM  is corrected using the optical loss Lb(λ) stored in said control means in accordance with the following equation:  
       
         
           P OUT (λ)=P OUTM (λ)/Lb(λ)  
         
       
       thereby obtaining a correct output light intensity P OUT (λ) from said optical fiber amplifier. 
     
     
       35. An optical amplifier evaluation apparatus according to claim  28 , wherein said control means applies output light from said unpolarized light generator to the input terminal to said first optical modulator, sets said first optical switch to the “steady state”, and connects said optical spectrum analyzer to the output terminal to said optical spectrum analyzer in the “steady state”, thereby measuring a light intensity Pa(λ) at each wavelength λ of the optical signal having passed from said optical fiber amplifier through an optical path including said first optical modulator and said first optical switch, 
       an optical loss La(λ) on the optical path is obtained and stored in said control means using the light intensity P ref (λ) of reference light stored in said control means in accordance with the following equation:  
       
         
           La(λ)=Pa(λ)/P ref (λ)  
         
       
       said control means applies output light from said unpolarized light generator to the input terminal to said first optical modulator, sets said first and second optical switches to the “switching state”, and connects said optical spectrum analyzer to the output terminal to said optical spectrum analyzer in the “switching state”, thereby measuring a light intensity Pd(λ) at each wavelength λ of the optical signal having passed from said optical fiber amplifier through an optical path including said first optical modulator and said first and second optical switches,  
       an optical loss Ld(λ) on the optical path is obtained and stored in said control means using the light intensity P ref (λ) of reference light, stored in said control means in accordance with the following equation:  
       
         
           Ld(λ)=Pd(λ)/P ref (λ)  
         
       
       said control means applies output light from said unpolarized light generator to the input terminal to said optical fiber amplifier, sets said first optical switch to the “steady state”, sets said second optical switch to the “switching state”, and connects said optical spectrum analyzer to the output terminal to said optical spectrum analyzer in this state, thereby measuring a light intensity Pb(λ) at each wavelength λ of the optical signal having passed from said optical fiber amplifier through an optical path including only said first and second optical switches,  
       an optical loss Lb(λ) on the optical path not including said second optical modulator is obtained and stored in said control means using the light intensity P ref (λ) of reference light stored in said control means in accordance with the following equation:  
       
         
           Lb(λ)=Pb(λ)/P ref (λ)  
         
       
       said control means applies output light from said unpolarized light generator to the input terminal to said optical fiber amplifier, sets said first and second optical switches to the “steady state”, and connects said optical spectrum analyzer to the output terminal to said optical spectrum analyzer in the “steady state”, thereby measuring a light intensity Pc(λ) at each wavelength λ of the optical signal having passed from said optical fiber amplifier through an optical path including said first and second optical switches and said second optical modulator, and  
       an optical loss Lc(λ) on the optical path including said second optical modulator is obtained and stored in said control means using the light intensity P ref (λ) of reference light stored in said control means in accordance with the following equation:  
       
         
           Lc(λ)=Pc(λ)/P ref (λ).  
         
       
     
     
       36. An optical amplifier evaluation apparatus according to claim  35 , wherein said control means sets said first and second optical switches to the “switching state”, and sends a light intensity measurement command to said optical spectrum analyzer in the “switching state” to analyze a spectrum of incident light, thereby obtaining a light intensity P INM  at each wavelength λ, 
       the light intensity P INM  is corrected using the optical losses Ld(λ) and La(λ) stored in said control means in accordance with the following equation:  
       
         
           P IN (λ)=P INM (λ)·La(λ)/Ld(λ)  
         
       
       thereby obtaining a correct output light intensity P IN (λ) from said optical fiber amplifier, 
       said control means sets said first optical switch to the “steady state”, sets said second optical switch to the “switching state”, and sends a light intensity measurement instruction to said optical spectrum analyzer in this state to spectrum-analyze incident light, thereby obtaining a light intensity P OUTM  at each wavelength λ,  
       the light intensity P OUTM  is corrected using the optical loss Lb(λ) stored in said control means in accordance with the following equation:  
       
         
           P OUT (λ)=P OUTM (λ)/Lb(λ)  
         
       
       thereby obtaining a correct output light intensity P OUT (λ) from said optical fiber amplifier, 
       said control means sets said first and second optical switches to the “steady state”, and sends a light intensity measurement instruction to said optical spectrum analyzer in the “steady state”, thereby obtaining a light intensity P ASEM  at each wavelength λ of an optical signal as spontaneous emission (ASE) in a partial period T A  of the OFF period of the input, amplified optical signal, and  
       the light intensity P ASEM  is corrected using the optical loss Lc(λ) stored in said control means in accordance with the following equation:  
       
         
           P ASE (λ)=P ASEM (λ)/Lc(λ)  
         
       
       thereby obtaining a correct input light intensity P ASE (λ) of spontaneous emission (ASE) in said optical fiber amplifier. 
     
     
       37. An optical amplifier evaluation apparatus according to claim  36 , wherein a gain G(λ) and noise figure NF(λ) of said optical fiber amplifier are calculated using the corrected input light intensity P IN (λ), output light intensity P OUT (λ), and light intensity P ASE (λ) of spontaneous emission (ASE) in accordance with following equations: 
       
         
           G(λ)=P OUT (λ)/P IN (λ),  
         
       
       
         
           NF(λ)=P ASE (λ)/[h·ν·G·Δν] 
         
       
       where 
       h: Planck's constant  
       ν: light frequency of input optical signal  
       G: gain  
       Δν: measurement frequency resolving power width (measurement frequency width) of light intensity measurement device.  
     
     
       38. An optical amplifier evaluation apparatus according to claim  37 , wherein, in the “steady state” by a command from said control means, said first optical switch inputs an optical signal having passed through the first and second terminals to an input terminal of said optical fiber amplifier via the output terminal to said optical fiber amplifier, and inputs the amplified optical signal output from an output terminal of said optical fiber amplifier to the third terminal via the input terminal from said optical fiber amplifier and then to the first terminal of said second switch via the fourth terminal, and 
       in the “steady state” by an instruction from said control means, said second optical switch inputs the optical signal input to the first terminal, to said second optical modulator via the second terminal and the input terminal to said second optical modulator, inputs the modulated optical signal output from said second optical modulator to the third terminal via the output terminal from said second optical modulator, and inputs the optical signal input to the third terminal, to said external optical spectrum analyzer serving as said light intensity measurement via the fourth terminal and the output terminal to said light intensity measurement device, thereby allowing to measure the light intensity P ASE  of spontaneous emission in said optical fiber amplifier by the first switching operation.  
     
     
       39. An optical amplifier evaluation method comprising: 
       modulating light output from a light source into a rectangular optical signal having predetermined ON and OFF periods by a first optical modulator;  
       applying the optical signal modulated by said first optical modulator to an optical fiber amplifier to be evaluated;  
       passing the optical signal output from said optical fiber amplifier to be evaluated through a second optical modulator only during a given period in the OFF period of the optical signal modulated by said first optical modulator, thereby measuring a light intensity P ASE  of spontaneous emission in said optical fiber amplifier by a light intensity measurement device;  
       obtaining an optical loss on an optical path extending from said optical fiber amplifier to said light intensity measurement device using output light from an unpolarized light generator, in a no-input state, and correcting, using the obtained optical loss, the light intensity P ASE  of spontaneous emission in said optical fiber amplifier that is measured by said light intensity measurement device; and  
       obtaining a noise figure NF of an optical signal in said optical fiber amplifier using a corrected light intensity P ASE ′ of spontaneous emission in said optical fiber amplifier in accordance with the following equation:  
       
         
           NF=P ASE ′/(h·ν·G·Δν)  
         
       
       where 
       h: Planck's constant  
       ν: light frequency of input optical signal  
       G: gain  
       Δν: measurement frequency resolving power width (measurement frequency width) of said light intensity measurement device.  
     
     
       40. An optical amplifier evaluation method according to claim  39 , further comprising: 
       measuring a light intensity in the ON period of the optical signal input to said optical fiber amplifier by said light intensity measurement device;  
       measuring a light intensity in the ON period of an optical signal output from said optical fiber amplifier by said light intensity measurement device; and  
       obtaining the gain G of said optical fiber amplifier from the light intensities in the ON periods of the optical signals input to and output from said optical fiber amplifier that are measured by said light intensity measurement device.